Lung cancer, which is the leading cause of cancer death in Japan, Europe, and the United States, is refractory. Lung cancer can be divided into small cell lung cancer and non-small cell lung cancer with prevalence rates of 15% and 85%, respectively, and non-small cell lung cancer can be histopathologically classified into three types: adenocarcinoma, squamous cell carcinoma, and large cell carcinoma.
Smoking is still the leading risk factor for lung cancer, but recently the portion of never smoker-related lung cancer (which is mainly categorized as adenocarcinoma) is increasing. Lung cancer patients have an overall 5-year survival rate of only about 15%. This is largely due to the lack of methods for detecting early stage lung cancer, and only 16% of patients are diagnosed with early-stage diseases. Current screening methods such as chest X-ray, sputum cytologic examination, and helical CT have not yet shown their effectiveness in the improvement of mortality of lung cancer.
On the other hand, serum biomarkers for lung cancer have been investigated to achieve early detection of the disease and improve clinical management (see Non-patent Literature 1). Nonetheless, their present clinical usefulness remains limited (see Non-patent Literatures 2 and 3). CEA (carcinoembryonic antigen) and CYFRA (cytokeratin 19 fragment) are elevated in sera from a subset of non-small cell lung cancer patients, and they are clinically available for monitoring the disease status and evaluating the response to the treatments. However, they are not recommended for use in clinical diagnosis (see Non-patent Literature 4), because they have been also found to be associated with other types of diseases such as smoking, lung inflammation, and other types of cancer, and they do not have the sufficient power to detect early-stage lung cancer.
Recently, monitoring the protein expression pattern in clinical specimens by proteomics technologies has offered great opportunities to discover potentially new biomarkers for the diagnosis of cancer. Various proteomic tools such as 2D-DIGE, SELDI-TOF-MS, protein arrays, ICAT, iTRAQ, and MudPIT have been used for differential analysis of various biological samples including cell lysates, serum, and plasma to better understand the molecular basis of cancer pathogenesis and the characterization of disease-associated proteins (see Non-patent Literature 5).
In order to identify minor components in complicated biological samples as putative biomarkers, focused proteomics or targeted proteomics technologies have been especially attracting attention. They involve, for instance, the phosphoproteomics technologies such as IMAC (see Non-patent Literature 6), the cell-surface-capturing (CSC) technology (see Non-patent Literatures 7 and 8), and glycan structure-specific quantification technology IGEL (see Non-patent Literature 9). These methods can circumvent the technological limitations that currently prohibit the sensitive and high-throughput profiling of, in particular, blood proteome samples because of their high complexity and large dynamic range of proteins. Similarly, a branch of proteomics dealing with naturally occurring peptides is often referred to as peptidomics. Direct analysis of peptides produced by processing or degradation of proteins might be useful for detecting biomarker peptides in body fluids (see Non-patent Literature 10). Further, as one sphere of peptidomics, a method for obtaining novel biomarkers with use of mass spectrometry technology has also been proposed (see Non-patent Literature 11).
So far, more than 500 proteases/peptidases are known to be expressed by human cells (see Non-patent Literatures 12 and 13). They function at almost all locations in the body including intracellular region, extracellular matrices, and in blood, involved in activation of other protein functions, degradation, of cellular proteins, and notably tumor progression or suppression (see Non-patent Literatures 14 to 16). Indeed, many matrix metalloproteases are overexpressed in various types of tumor cells, which facilitate construction of favorable micro-environment for tumor cells and promote metastasis (see Non-patent Literature 16).